A sand and gravel operation reported that a Komatsu WA470-6 wheel loader was slipping out of gear under heavy load and taking over five seconds to engage reverse. The transmission temperature gauge was consistently reading near the red zone, and the operators noted a shuddering vibration when traveling at high speed on the haul road.
The diagnostic process began by checking the transmission oil level and condition. The oil was dark brown and smelled heavily burnt, indicating severe thermal breakdown. The technician dropped the transmission sump pan. The bottom was covered in a thick layer of black, gritty sludge, and the magnetic pickup plug was covered in fine metal shavings. The suction screen was completely occluded by a mat of friction material fibers.
Because the machine had high-speed shudder, the technician suspected the torque converter lock-up clutch. The WA470-6 uses a lock-up clutch to provide direct drive in higher gears, improving fuel efficiency. A pressure test was performed on the lock-up circuit. The pressure was erratic, fluctuating between 100 and 300 psi, far below the specification of 600 psi.
The torque converter was removed and cut open. The lock-up clutch friction plates were worn down to the steel plates, and the piston seals had hardened and cracked. The slipping lock-up clutch was generating massive friction heat during high-speed travel, which burned the transmission fluid. The burnt fluid lost its lubricity, causing the forward and reverse clutch packs in the transmission to slip and engage slowly.
The fine metal shavings were traced back to a failing front pump. The pump housing was scored, reducing the overall line pressure to the entire transmission, compounding the clutch engagement issues.
The repair involved installing a remanufactured torque converter and a new front hydraulic pump. The transmission was flushed multiple times with a solvent fluid to remove the varnish from the valve body. The external transmission oil cooler was replaced, as the burnt fluid had left a tenacious insulating layer inside the cooler tubes. After refilling with fresh SAE 10W transmission oil and adjusting the inching pedal, the loader engaged gears instantly, the shudder vanished, and the temperature held steady at 85°C.
